Chemical vapor deposition processes such as pyrolytic processes and hydrolytic processes are well known in art of coating substrates. The physical characteristics of the coating reactants utilized in such processes may be liquid, vapor, liquids or solids dispersed in gaseous mixtures, aerosols, or vaporized or vaporous coating reactants dispersed in gaseous mixtures.
In the process of deposition of a vaporized chemical compound on a glass substrate in the production of photovoltaic devices, the vaporized chemical compound is typically deposited in a vacuum atmosphere. The systems for carrying out such process have typically included a housing having an enclosed deposition chamber formed of a lower portion and an upper portion with a horizontal junction with each other. A seal assembly is interposed at the junction between the lower and upper housings. A conveyor means is provided to transport glass sheet substrates through the chamber. A chemical vapor distributor is located within the deposition chamber to provide a coating on the glass substrate as the substrate passes through the chamber.
The system includes a vacuum source for drawing a vacuum within the deposition chamber. The deposition chamber typically includes elongate heaters for heating the glass sheets as they are conveyed through the system. The glass sheets pass into the deposition chamber from a vacuum-heating furnace to the vacuum deposition chamber that is maintained at a similar vacuum and temperature setting as the heating furnace. Powdered cadmium sulfide and powdered cadmium telluride are fed into the vaporization deposition chamber. The films are then deposited onto the previously coated and heated glass substrates sequentially. The coated substrates are next transferred through a load lock and thence into a cooling chamber wherein cooling is effected by compressed nitrogen and finally conveyed to atmosphere pressure though an exit load lock to an air cooling section for reduction to ambient temperature. The cadmium telluride thin-film material requires a follow-on processing step to re-crystallize its polycrystalline structure so that effective photovoltaic devices can be made from the film stack. Typically this step is accomplished by applying a solution of cadmium chloride to the cadmium telluride surface of the cooled coated glass and re-heating the glass to a temperature of about 390° C. to 420° C. for a period of about 15 to 20 minutes. Care must be taken to slowly heat and cool the glass to avoid breakage during this treatment which extends the overall process time of the required step.
Since it is well recognized that renewable energy sources are becoming increasingly more important, it is deemed that commercial production of photovoltaic devices for the generation of electrical energy is important in satisfying the renewable energy needs. The utilization of thin-film coatings of semiconductor materials on glass substrates is considered to be a viable mechanism in the field of photovoltaic-based electrical energy generation systems.
It has been found that thin-film coating systems, based upon the above referred to technology, are capable of depositing thin film of cadmium sulfide/cadmium telluride photovoltaic material onto commercially available soda-lime glass substrates in a vacuum. The photovoltaic materials are subsequently treated to re-crystallize the cadmium telluride surface making the film stack ready for further processing into photovoltaic devices. While the above-described system is capable of producing photovoltaic panels suitable for the production of electrical energy, it would be desirable to reduce the cost for such production to render the system commercially viable.